单项选择题

有以下程序:#include <iostream>#include <math>using namespace std;class point{private: double x; double y;public: point(double a, double b { x=a; y=b; friend double distance (point a, point b ; };double distance(point a, point b return sqrt((a. x-b. x )*(a. x -b. x )+ (a. x -b. x)*(a. x-b. x));}int main (){ point p1(1,2); point p2(5,2); cout<<distance (p1, p2)<<end1; return 0;} 程序运行后的输出结果是

A.1
B.5
C.4
D.6